function delay (str) {
    return new Promise(function (resolve, reject) {
        setTimeout(() => {
            resolve(str);
        }, 10);
    }) 
} 

delay().then(()=>{
    var a ='hello '
    return delay(a)
}).then(value=>{
    var b='lagou '
    return delay(value+b)
}).then(value=>{
    var c = ' I love U'
    console.log(value+c);  // hello lagou i love u
})
